1. Field of the Invention
The present invention relates to a measurement method and measurement apparatus.
2. Description of the Related Art
An optical observation apparatus (for example, astronomical telescope) for an observation target whose position is not constant generally changes its attitude to track the observation target. The direction of gravity is constant regardless of the attitude of the observation apparatus. As the attitude of the observation apparatus changes, deformation of the optical system of the observation apparatus by the gravity changes (that is, the optical performance of the optical system changes).
The optical system, which forms the observation apparatus such as an astronomical telescope, is roughly divided into a primary mirror that collects observation light, and a correction optical system that corrects aberration. A change of the optical performance of the optical system caused by a change of the attitude of the observation apparatus is obtained in advance. Then, for example, the relative distances between a plurality of optical elements which form the correction optical system are adjusted. The optical performance of the observation apparatus can therefore be adjusted to a predetermined value. To adjust the optical performance of the observation apparatus with high precision, it is indispensable to accurately measure the optical performance (for example, wavefront aberration (transmission wavefront aberration)) of the correction optical system.
An interferometer is generally used to measure the wavefront aberration of the optical system (see Japanese Patent Laid-Open No. 2002-214076). In Japanese Patent Laid-Open No. 2002-214076, a reference surface, an optical system to be measured, and a reflecting surface are arranged to make their optical axes coincide with each other. The wavefront aberration of the optical system to be measured can be measured by detecting the interference pattern of light which has been reflected by the reference surface, and one which has passed through the reference surface and the optical system to be measured and has been reflected by the reflecting surface. At this time, the light reflected by the reflecting surface contains a wavefront unique to the reference surface, the gravitational deformation of the reference surface, a wavefront unique to the reflecting surface, and the gravitational deformation of the reflecting surface. A wavefront error unique to the interferometer other than that caused by the optical system to be measured will be defined as the system error of the interferometer (measurement apparatus). In Japanese Patent Laid-Open No. 2002-214076, a system error arising from the reflecting surface is calibrated (separated) by rotating the reflecting surface about its optical axis. When a system error arising from the reference surface and that arising from the reflecting surface are separated, correction can be done at different magnifications for the NA (Numerical Aperture) of the reference surface and that of the reflecting surface. Hence, the system error can be calibrated with higher precision.
A technique has also been proposed, to hold an optical element so that its optical axis becomes perpendicular to the direction of gravity, and measure the surface shape of the optical element (see Japanese Patent Laid-Open No. 11-325818). In Japanese Patent Laid-Open No. 11-325818, an optical element to be measured and a reference optical element are suspended using belts to reduce the gravitational deformations of the optical elements depending on the attitude.
It is also reported that loads having a cosine distribution along the normal of an optical element are added to the periphery of the optical element to cancel the in-plane gravity component of the optical element, thereby reducing the gravitational deformation of the optical element (see Schwesinger G., “Optical Effect of Flexure in Vertically Mounted Precision Mirrors”, Journal of the Optical Society of America, Vol. 44, pp. 417-424, 1954 (reference 1)).
However, in Japanese Patent Laid-Open No. 2002-214076, when the optical axis of a measurement apparatus (optical system to be measured) does not coincide with the direction of gravity, the system error cannot be calibrated. This is because even when the reflecting surface is rotated, the gravitational deformation components of the reference surface and reflecting surface do not follow the rotation of the reflecting surface.
When an optical element is suspended using a belt, like Japanese Patent Laid-Open No. 11-325818, uniform loads are added along the normal of the optical element to a periphery (lower half) in the direction of gravity out of the periphery of the optical element. It is difficult to say that the in-plane gravitational deformation of the optical element is effectively reduced. Note that the purpose of Japanese Patent Laid-Open No. 11-325818 is to reduce the gravitational deformation of an optical element when the optical element is held so that its optical axis becomes perpendicular to the direction of gravity. This technique cannot be applied to a case in which the optical axis forms various angles with respect to the direction of gravity, like a correction optical system used in an observation apparatus such as an astronomical telescope.
Reference 1 does not disclose a concrete arrangement for adding loads having a cosine distribution along the normal of an optical element. This technique cannot be directly applied to, for example, measurement of the wavefront aberration of the correction optical system used in the observation apparatus.